In many types of combustion operations, acid gases are formed and these must be reduced to non-harmful or regulated levels prior to emission of the combustion gases to the atmosphere. Such acid gases include the sulfur oxides and the halogen acids such as hydrochloric, hydrofluoric, hydrobromic and hydroiodic acid. The acid gases are generated in the combustion of coal and fuel oil and in the incineration of municipal garbage, hazardous wastes and/or other wastes which may typically contain halogenated compounds in the form of solvents, scrap plastics, and the like.
There are three major types of prior art methods for the removal of acid gas components from flue gases: wet scrubbing, spray-drying (so-called "wet-dry" scrubbing) and dry solids contacting. Both wet scrubbing and spray-drying processes suffer from major corrosion, plugging and scaling problems associated with the presence of an aqueous solution phase. There is some confusion in existing terminology resulting from the fact that some proponents of spray-drying technology have wishfully labeled spray-drying operations as "dry" processes; as used herein, spray-drying is considered to be a wet-dry process. Economically, true dry solids processes require much lower capital investments than either wet scrubbing or spray-drying processes. However, although dry solids contacting methods avoid the problems of the wet methods, the acid gas removal efficiency obtained in these processes is generally lower than in wet processes because of slower gas-solid reaction kinetics. Methods for improving the reaction kinetics of true dry solids processes are required to make these processes technically and commercially feasible for the removal of acid gases.
It is known in the art to employ hygroscopic or deliquescent additives in conjunction with flue gas humidification/cooling for both dry and wet-dry (spray-drying) SO.sub.2 removal processes in order to improve acid-removal efficiency. Karlsson, et al, in the Journal of the Air Pollution Control Association, pp.23-28, Vol.33, No. 1, January, 1983, theorize that the characteristic of such materials in spray drying operations is to retain liquid beyond the normal drying times and prolong the period during which ionic reactions, i.e., neutralization of absorbed acid gases with dissolved alkaline reactant in solution, can occur. Karlsson explored a large number of compounds as reaction enhancement additives in the wet-dry scrubbing of SO.sub.2. It was found that the most effective compounds were deliquescent at the conditions employed, 70.degree. C. and 61% relative humidity, which are not conditions that prevail in flue gas. In order to achieve the conditions necessary for liquid retention by these deliquescent compounds, the flue gas must be both cooled and humidified, a process that has serious side-effects.
Similarly, Lindau and Ahman, in U.S. Pat. No. 4,454,102, claim the use of hygroscopic materials to form a liquid phase partially covering the surface of the alkaline solids collected on a baghouse filter in a spray-drying process for acid gas removal from flue gas. Lindau and Ahman typically operate the baghouse filter at temperatures of 70.degree. C., which flue gas temperature level must be achieved by evaporative cooling, a process with severe potential problems in an acid-gas containing flue gas. Further, the compounds of Lindau and Ahman are hygroscopic compounds, not deliquescent ones.
Additional prior art that calls for cooling and conditioning the hot flue gas to increase the degree of water vapor saturation include Shale and Cross in U.S. Pat. No. 3,976,747, for a dry process, and Felsvang, et al, in U.S. Pat. No. 4,279,873, for a spray-drying process. This and other prior art call for evaporative cooling by water sprays to achieve a close approach to gas saturation. Cooling of the gas to near-saturation temperatures has a number of distinct disadvantages which render this mode of operation unattractive in a continuous industrial operating situation. P. S. Farber, in an article, "Emissions Control Through Dry Scrubbing", Environmental Progress, Vol. 5, No. 3, pp. 178-183, August, 1968, discusses the use of spray drying methods of SO.sub.2 removal from flue gases. Farber states that it is desirable to maintain the combustion gas as close to its dewpoint as possible to facilitate the time and rate of transfer of the acid gases to the available alkali. However, to avoid forming mud on the bag filters or on the plates of the downstream ESP, or the severe corrosive effects of condensation in an acid gas system, the approach is "normally controlled at no less than 19.degree. F. above the dewpoint". Such exact control is not only difficult to maintain, but cooling the gas to temperatures approaching the dewpoint has additional consequences other than the undesirable and serious problems indicated by Farber. Cooling of the gas causes loss of plume buoyancy with the resulting possibility of forming local ground fog, reduced visibility and associated hazards. Restoration of plume buoyancy requires reheating the flue gases and the wasteful use of heat energy.
Additionally, cooling of a gas containing such acid components as HCl, SO.sub.2 and/or SO.sub.3 close to its dewpoint gives rise to severe corrosive conditions. If there are even minor concentrations of HCl or SO.sub.3 present, the dewpoint will be dramatically elevated. For example, a gas with a 1000 ppmv content of HCl gas with a water wet bulb temperature of 140.degree. F. (dewpoint of about 120.degree. F.) will have an acid dewpoint of 148.degree. F., with a liquid-phase HCl concentration of 14% by weight. For SO.sub.2 /SO.sub.3 the situation is much worse because of the higher boiling points of sulfuric acid. A flue gas with a water concentration of 12% has a dewpoint of approximately 120.degree. F. in the absence of SO.sub.3. The addition of only 1 ppm SO.sub.3 raises the dewpoint to 230.degree. F. In the latter case, it is not necessary to cool to the water dewpoint to generate acid formation and corrosive attack. The presence of small amounts of SO.sub.3 is virtually inescapable when burning a high-sulfur fuel or a variable waste material that contains sulfur or H.sub.2 SO.sub.4. To avoid corrosion problems and the possible loss of the downstream equipment, it is necessary to keep the gas hot and well above the acid dewpoint. For a gas containing traces of SO.sub.3, cooling to a controlled approach to the much lower water wet bulb temperature, as taught by the prior art, can result in catastrophic corrosion failure.
Yoon, in U.S. Pat. No. 4,604,269, removes sulfur oxides from flue gas by first cooling the flue gas to "a relatively low temperature", contacting with a finely divided dry sorbent which has been treated with a solubilizing solution, and then humidifying the flue gas with a water spray or steam injection. Yoon conveys the treated solids to the flue gas injection point using air or superheated steam. Superheated steam is by definition at temperatures above the boiling point of water at its specific pressure, and such steam is "dry" and cannot effect deliquescence. Yoon therefore effects the liquefaction of his "solubilizing compounds" after injection into the flue gas by humidifying the flue gas with water sprays or steam. Again, water spraying is the equivalent of the wet-dry spray drying-operation, with all of the disadvantages of wet processes, and injection of steam into bulk flue gas for purposes of humidification is highly uneconomic because of the large quantities of steam required to humidify the relatively very large total flue gas flows.